Electricity meters are routinely used to measure power and energy consumption. For example, electrical utilities utilize electricity meters to measure energy and power consumption for subscriber billing and power management purposes. Electricity meters can also be designed to measure and record the electrical energy used by one or more specific loads connected to an electrical network, for example the mains power supply in commercial or residential premises. These types of devices can also be designed to measure and record other characteristics of electrical supply and usage such as brownouts and overvoltages, heavy usage intervals, etc.
Many electricity meters are microprocessor-based and are powered by a low voltage adapter, which does not present a significant fire or shock hazard. Such an electricity meter thus forms part of a low-energy power circuit which operates on a “Class 2” power supply and falls into the general class “information technology equipment”. For the purposes contained herein the term “electricity meter” shall mean an electricity meter which is powered by a Class 2 power supply or battery. The terms “Class 1 power circuit” and “Class 2 power supply” are defined in the national electrical code.
The safety features required in an appliance powered by a Class 2 power supply are considerably less stringent than those required by an appliance that is powered by a higher voltage “Class 1” power circuit, for example the mains power supply of a commercial or residential premises. A Class 1 power circuit has a sufficiently high voltage and is capable of delivering sufficient energy as to present a significant fire and shock hazard, and any appliance powered by a Class 1 power circuit must be designed so that no single failure will result in a shock or fire hazard. An appliance connected to a Class 1 power circuit may for example be properly grounded with all conductive components suitably insulated against exposure to physical contact (known as a “Class I” appliance), so that either the ground or the insulation will provide protection against electrical shock and fire in case one or the other of these safeguards fails. Alternatively, an appliance connected to a Class 1 power circuit may be double insulated (known as a “Class II” appliance), each layer of insulation providing independent protection against exposure to physical contact, so that in the event that one layer of insulation fails the other layer of insulation will still provide protection against electrical shock and fire. These types of safeguards add considerable cost to the manufacture of the appliance.
An electricity meter, typically powered by a Class 2 power supply in the order of 6V and a few hundred mA, is an example of a Class III appliance. Class III appliances operate at such low voltages that they do not present the same shock and fire hazards as appliances that are connected to a higher voltage Class 1 power circuit. As such, Class III appliances have relatively few safety requirements in comparison to Class I and Class II appliances, which considerably reduces the manufacturing costs of a Class III appliance.
One method in which an electricity meter monitors current in a conductor of a Class 1 power circuit uses milli amp current transformers, which are inductively coupled to the power circuit conductors. Such sensors output low current signals, linearly scaled down and proportional to the current flowing through on the conductor with a maximum current output in the order of 20 mA to 100 mA. When these low current signals are converted to voltages using burden resistors or other means such as a current-to-voltage converter circuit, the maximum voltages are in the order 0.2V to 1.2V and well within the voltage levels of a Class 2 power circuit. However, although it is powered by a Class 2 power supply, because its operation involves inductive components (sensors) that are coupled to a Class 1 power circuit, an electricity meter must be manufactured to the standards of an appliance operating off of a Class 1 power circuit. Even though these sensors are not directly coupled to the power circuit conductor in normal operation, in the event of a fault condition whereby the sensor winding becomes conductively coupled to the conductor being monitored, the sensor conductors feeding the input of the electricity meter would become energized to Class 1 voltage levels, which would in turn energize the electricity meter to Class 1 power circuit voltage levels.
Accordingly, even though an electricity meter operates on a Class 2 power supply and receives low voltage input signals from sensors such as current transformers and voltage transformers to take measurements, there is a possibility that a Class 1 power circuit being monitored may come into conductive contact with a sensor, for example in the case of insulation failure. Therefore, for safety reasons such an electricity meter is treated as though it is part of a Class 1 power circuit, and all safety features required in appliances powered by a Class 1 power circuit are required to be built into the electricity meter even though such safety features are unnecessary in its normal operation.
Compliance with the protective measures required in an appliance connected to a Class 1 power circuit adds considerable cost to the manufacture of an electricity meter. It would accordingly be advantageous to provide an interface which allows an electricity meter to be designed and constructed as an appliance operating in a Class 2 power supply, avoiding the costly protective measures required in appliances that operate as part of a Class 1 power circuit while still complying with all safety requirements.